This invention relates to an optical apparatus in general and more particularly to an optical apparatus for detecting cavitation bubbles in liquids.
Cavitation occurs when shock waves of sufficiently low pressure pass through a liquid. It also occurs in extremely fast liquid flows in the liquid itself, or it may occur on boundary surfaces. The cavitation bubbles that form collapse after a short time. When they collapse, damage may occur on the surfaces on which the bubbles have settled. To avoid undesirable damage, it is necessary to detect the formation and progress of these cavitation bubbles and to monitor them.
An example where the use of ultrasonic shock waves with steep pulse edges leads to the formation of cavitation bubbles is found in lithotripters. Lithotripters are used to disintegrate calculi, such as kidney stones, which are found in the body of a patient. Therefore, one must monitor the occurrence of cavitation effects in the sonic field, which in this example means they must be monitored inside the body.
One way of detecting cavitation bubbles is by direct optical observation with the aid of a high-speed camera. Such a method is costly, however, and can only be used for optically transparent liquids. Also, it is not practical for applications involving continuous operation.
An apparatus for detecting cavitation bubbles is disclosed in Collapse of Multiple Gas Bubbles by a Shock Wave and Induced Impulsive Pressure, Y. Tomita et al., J. Appl. Phys. 56 (1), 1984, pp 125 to 131, where the ultrasonic pulse emitted by a collapsing cavitation bubble is measured. However, this method is not suitable for lithotripters, because the ultrasonic pulse that is emitted has only a small amplitude and it is difficult to separate from the reflected shock wave which is damping out.
In view of the prior art, there is a need for an apparatus for detecting cavitation bubbles in liquids that is inexpensive and with which it is possible to prove with certainty the existence of cavitation bubbles, even in the presence of a disturbing ultrasonic background.